I prefer playing with keyboard, but maybe mouse is the way a game like this has to go.
Its crippled by the fact that its an online game, and that its made in GM, so really the faster option would be a needed compromsie for it being an online game.
You could try out: press a key, and the player keeps moving in that direction until another key is pressed.
If the opposite direction is pressed, the player stops: Press it again, move in opposite direction.
If up is pressed when already moving left, move up etc.
What i mean is like, pressing a key will add that kets value onto your speed on that keys axis, to a maximum of -S,+S, and of course, diagonal will be easy to detect, and the speed reduced accordingly.
It would be strange, but you could think about it? In addition, the numberpad could be an alternative option for thatl, where pressing '5' stops
motion whichever direction your going in.
On the other hand, mouse movement gives people the power to click where they want to go, and not just the direction they want to go in....hmph...